Help: Add notes for topic 'swig_add_library'

Help/release/dev/swig_add_library.rst

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+swig_add_library
+----------------
+
+* The :module:`UseSWIG` module gained a ``swig_add_library`` command
+  to give more flexibility over the old ``swig_add_module`` command.
+
+* The :module:`UseSWIG` module ``swig_add_module`` command is now
+  deprecated in favor of ``swig_add_library``.